1. Field of the Invention
This invention relates fluorescent lamps, and more particularly to methods of manufacturing a fluorescent lamp having metal oxide thin film layers on the inner surface of the fluorescent lamp wall.
2. Statement of the Problem
A typical fluorescent lamp comprises a cylindrical glass tube or envelope containing mercury vapor and a phosphor layer covering the inside of the tube wall. Many fluorescent lamps, in particular rapid-start fluorescent lamps, usually comprise one or more metal oxide layers; for example, an electrically conductive metal oxide layer on the inner surface of the glass tube, and a metal oxide protective layer between the conductive layer and the phosphor layer of the lamp. A conventional technique of the art of forming metal oxide layers in fluorescent lamps involves: dispersing a solid powder of the desired oxide in a liquid medium to make a colloidal suspension of the oxide; applying a coating of the suspension onto a surface of the lamp; and drying the coating to form the oxide layer. Generally, it is difficult to achieve a uniform, continuous thin film by applying a colloidal suspension of powdered particles. Another technique involves dissolving a precursor compound in a solvent and spraying the precursor solution onto a hot surface having a temperature above the crystallization temperature of the desired oxide, whereby the precursor compound is immediately pyrolyzed. A typical conventional precursor for a conductive layer contains tin tetrachloride, SnCl.sub.4, and hydrogen fluoride, HF. The chlorine and fluorine are highly electronegative, salt-forming atoms that may lead to lamp defects called "measles", as described below. Further, highly reactive precursor compounds such as SnCl.sub.4 and HF are toxic and difficult to handle, and do not store well. A typical conventional precursor for a protective layer is a metal alkyl compound in a solvent. It is generally difficult to form a uniform, continuous metal oxide thin film by the conventional pyrolysis method of the prior art because pyrolysis of the sprayed precursor compound on the hot substrate results in a broken, uneven surface on the microscopic level.
Fluorescent lamps are subject to the formation of localized defects called "black spot patches" or "measles". A conductive layer is usually located between the inner surface of the glass tube wall and the phosphor layer. The conductive layer is a conductive metal oxide, such as tin oxide or indium oxide. The conductive layer serves to reduce the voltage necessary for ignition of a fluorescent lamp. Measles are believed to develop during lamp operation as a result of an interaction involving the conductive layer and the mercury in the arc discharge. The mercury is presumed to penetrate the phosphor layer, leading to conditions that allow build-up of charge and subsequent discharge, which result in the measle defect by disrupting the phosphor layer and generally forming a small crater in the glass tube. In particular, the formation of measles is believed to be caused by the presence of salts in the conductive layer. The conductive layer, also called the "nesa", is conventionally prepared by spraying a chlorine-based liquid precursor, such as tin tetrachloride and HF in butanol, onto the inside wall of the glass tube envelope and pyrolizing the precursor to form the conductive layer on the inside surface of the glass tube. Due to the presence of chlorine or other electronegative atoms, the precursor reacts with sodium in the glass, forming salts. The salts act as holes on the surface of the conductive oxide layer and become starting points of arc discharge during operation of the fluorescent lamp. The dark arc spots, or measles, become destructive holes in the phosphor layer that shorten tube life. It is also known in the art to add small amounts of electronegative dopants, such as fluorine, to the conductive layer to increase its conductivity. This results in formation of the salts and measles.
It is known in the art to employ a protective layer of aluminum oxide, often called alumina, or certain other metal oxides, such oxides of cerium, yttrium, titanium, and zirconium to inhibit or delay discoloration and other appearance defects in the phosphor layer or the conductive oxide layer. Silicon oxide, often referred to as silica may also be included in a protective layer. These barrier layers of the prior art are located between the conductive oxide layer and the phosphor layer. The advantages of the protective coating are probably a result of the relatively nonporous metal oxide coating that protects the conductive oxide layer from ion bombardment resulting from arc discharge. Even though the phosphor layer overlies the conductive oxide layer, and is much thicker than the protective layer, it does not protect the conductive oxide layer. This is probably because the phosphor layer is more porous and less electrically insulating than the metal oxide protective layer. Nevertheless, such protective layers of metal oxides have not effectively prevented or reduced the occurrence of measle defects. Furthermore, the protective layer is generally formed utilizing aqueous colloidal suspensions or dispersion of the metal oxide in a liquid. As indicated above, it is difficult to deposit a continuous solid layer using a colloidal suspension or dispersion. Also, adding a protective layer between the conductive oxide layer and the phosphor layer necessarily increases the complexity and expense of manufacturing.
It is also known in the prior art to vary the resistivity of the conductive oxide layer to reduce the occurrence of measles. Typically, the resistivity in the conductive layer is designed to have a U-shaped profile, in which the resistance is high at the two ends of the glass tube, and low towards the center of the tube. The low resistance portion allows the flourescent lamp to obtain the benefits of a rapid-start, energy efficient lamp, while the high resistance in the end portions reduces the problem of measle defects. Typically, the low resistance portion near the center of the tube has a resistivity of about 10 k.OMEGA./square; the end portions typically have a resistivity in the range of 100-150 k.OMEGA./square. Conventionally, the U-shaped resistance profile is achieved during manufacture of the lamp by making the conductive oxide coating thicker at the ends of the lamp than at the middle. But the relative differences in electrical resistivity of conductive coatings produced in such a manner tend to decrease after about the first 500 hours of operation. Therefore, the occurrence of measle defects in lamps having varied thickness of the conductive oxide is merely delayed from a time following the first 1000 hours of operation to a later time after about 3000 to 4000 hours of operation. This is a short improvement in the total potential life of a fluorescent lamp, which is on the order of about 20,000 hours. Further, the extra process steps required to make a conductive layer having varying thicknesses along its axial length are complex and the results are not reproduced reliably.